1. Field of the Invention
The present invention relates to a method of continuously producing a lactam from an oxime in high-temperature high-pressure water, and more particularly to a novel continuous production method of continuously producing a lactam by carrying out a rearrangement reaction of an oxime in high-temperature high-pressure water using an acid. Yet more specifically, the present invention provides a method that is preferable and useful as an industrial technique, enabling production of a lactam without the need for treatment to neutralize a large amount of used waste sulfuric acid as seen in the case of a conventional production method in which fuming sulfuric acid is used as a catalyst.
2. Description of the Related Art
Conventionally, a lactam such as ε-caprolactam, which is used as a raw material of nylon 6, is industrially produced by a Beckmann rearrangement from a carbonyl compound oxime such as cyclohexanone oxime. A highly concentrated acid catalyst such as fuming sulfuric acid is used in this rearrangement reaction, and because the reaction is carried out while boiling, hydrolysis of the oxime will be brought about by the presence of even a very small amount of water in the system, resulting in a drop in the yield of the lactam. To prevent this, the usual method is to carry out the reaction while boiling, using fuming sulfuric acid as the acid catalyst. With this method, because the reaction is carried out under harsh conditions using fuming sulfuric acid, there are known to be problems with regard to corrosion of the materials of the apparatus, the hazardousness of the production process, and processing of the byproduct ammonium sulfate. When recovering the lactam, the sulfuric acid used must be neutralized with ammonia, and more than 2 kg of ammonium sulfate is produced as a byproduct per 1 kg of the lactam. Ammonium sulfate has little commercial value, and hence using the ammonium sulfate has become difficult, and thus it has become necessary to process the ammonium sulfate.
In recent years, fears over deterioration of the global environment have heightened, and in the chemical industry there have been calls for the development of environmentally friendly chemical processes that are simple and efficient, according to which reaction can be completed within a short time, and according to which hazardous substances are not used. Regarding lactam production, there have been calls for the development of a novel production process that is efficient, and does not use fuming sulfuric acid, with which there are problems in terms of corrosion of the materials of the apparatus, operational safety, and the environment.
As methods for resolving the above problems, two methods have been proposed in which reaction is carried out in high-temperature high-pressure water and an acid catalyst such as fuming sulfuric acid is not used at all, namely (1) a batch type synthesis method (O. Sato, Y. Ikushima and T. Yokoyama, Journal of Organic Chemistry 1998, 63, 9100–9102), and (2) a flow type synthesis method (Y. Ikushima, K. Hatakeda, O. Sato, T. Yokoyama and M. Arai, Journal of the American Chemical Society 2000, 122, 1908–1918).
With the batch type synthesis method (1), cyclohexanone oxime is sealed in a stainless steel tube of internal volume 10 ml, the stainless steel tube is put into a salt bath to raise the temperature thereof to 200 to 400° C. within 20 to 30 seconds, and reaction is carried out for 3 minutes to obtain the product. It is considered that this method is not suitable as a mass production process, but nevertheless the method has attracted attention as a synthesis method that does not use a highly concentrated acid catalyst such as fuming sulfuric acid. Because the method is operated intermittently in order to bring the reaction to completion one batch at a time, a time of approximately 20 to 30 seconds is required to raise the temperature to the set reaction temperature. There is thus a drawback in that a large amount of the hydrolysis product cyclohexanone tends to be produced while the temperature is being raised, and hence the yield of the targeted ε-caprolactam is quite low. Moreover, cyclohexanone is a raw material of cyclohexanone oxime, and hence the reaction goes in the reverse direction, which is a fatal drawback for an industrial process.
With the flow type synthesis method (2), on the other hand, the method is operated continuously and hence it is considered that the method is suitable for mass production, but a cyclohexanone oxime aqueous solution at room temperature is heated to produce the high-temperature high-pressure carrier water, and hence one would imagine that raising the temperature up to the set reaction temperature must take a long time. Accordingly, in an experiment in which reaction was carried out for 113 seconds under conditions of 350° C. and 22.1 MPa, the result was that only cyclohexanone was obtained as a product, with ε-caprolactam not being produced at all. Moreover, it is reported that even at 374.5° C., ε-caprolactam and cyclohexanone are both produced. Consequently, it seems that with this method, as with the batch type synthesis method (1), raising the temperature takes time, and hence a large amount of cyclohexanone is produced through hydrolysis of the cyclohexanone oxime while the temperature of the solvent water is passing from, for example, 100 to 300° C., and thus there is a drawback in that the yield of the targeted ε-caprolactam is reduced.